Power supply, or ballast, circuits for various loads typically include a converter for supplying a.c. or d.c. current to a load. Such circuits typically include a pair of non-complementary switches in the converter. For example, it is common to use a pair of identical, n-channel enhancement mode MOSFETs as the switches. However, the use of such non-complementary MOSFETs has various drawbacks.
For instance, each of the non-complementary MOSFETs must be controlled by a separate gate-to-source (or control) voltage. This requires level shifting of voltage to couple a single control signal to each of the gate-to-source voltages of the pair of MOSFETs. Such level shifting can be accomplished by a transformer or by conventional bootstrapping means. The transformer method works well at high speeds, e.g., over 20 kilohertz, but is costly and hard to control. The bootstrapping method, usually implemented by an Integrated Circuit (IC), has good control capability, but is unable to work at high speeds, beyond 100 kilohertz.
In accordance with a first aspect of the invention, it would be desirable to provide a ballast circuit for a gas discharge lamp that overcomes the foregoing drawbacks. In accordance with a second aspect of the invention, it would further be desirable for such a ballast circuit to provide a feedback function to synchronize the frequency of lamp current to desired starting and operating frequencies, and also to provide a cathode preheat function. In accordance with a third aspect of the invention, it would be desirable to incorporate with either the first or second aspects of the invention circuitry for achieving a high power factor.